The present invention relates generally to the art of monitoring a wafer""s surface during a planarization process, and more particularly, relates to an improved apparatus whereby a plurality of electrical probes are used to detect endpoint during a chemical-mechanical planarization process.
Chemical-mechanical planarization (CMP) is now common in the manufacturing process of semiconductors. Semiconductors are constructed of multiple layers of material deposited on a wafer. CMP may be used throughout the manufacturing process of semiconductors to planarize the various layers of material deposited on the wafer. For example, CMP may be used to planarize the surface of a raw wafer or the surface of the wafer after a thin dielectric or conductive layer of material has been deposited on the wafer. The need for wafers to have a planar surface during the manufacturing process continues to increase as the size of the devices and interconnects used to manufacture semiconductors continues to decrease.
CMP is generally accomplished by pressing a surface of the wafer against a polishing pad attached to a rotating or orbiting platen in the presence of slurry. During the planarization process, it is desirable to gather data on the condition of the wafer""s surface. The data may then be used to optimize the planarization process or to determine when the planarization process should be terminated (referred to as endpoint).
It is generally preferred for endpoint systems to be in-situ systems to provide endpoint during the polishing process. Numerous in-situ endpoint systems have been proposed, but few have been successful in a manufacturing environment and even fewer are sufficiently robust for routine production use.
One approach that has proven production worthy is to measure changes in the friction between the wafer being polished and the working surface (generally a polishing pad). Such measurements are made by sensing changes in the motor current for the polishing platen. These systems use a global approach, i.e. the measured signal assesses the entire wafer surface. Thus, these systems do not obtain specific data about localized regions. Further, these endpoint systems work better for tungsten metal CMP because of the dissimilar coefficient of friction between the polish pad and the tungsten-titanium nitride-titanium film stack versus the polish pad and the dielectric underneath the metal. However, with advanced interconnection conductors, such as copper (Cu), the associated barrier metals, e.g. tantalum or tantalum nitride, may have a coefficient of friction that is similar to the underlying dielectric. The change in motor current, when transitioning between materials with similar coefficients of frictions, will be very small and difficult to detect. In addition, the motor current approach relies on detecting the Cu-tantalum nitride transition, then adding an over-polish time. Intrinsic process variations in the thickness and composition of the remaining film stack layer means that the final endpoint trigger time may be less precise than desirable.
Another group of approaches for detecting endpoint uses an acoustic approach. In a first acoustic approach, an acoustic transducer generates an acoustic signal that propagates through the surface layer(s) of the wafer being polished. Some reflection occurs at the interface between the layers, and a sensor positioned to detect the reflected signals can be used to determine the thickness of the topmost layer as it is polished. In a second acoustic approach, an acoustical sensor is used to detect the acoustical signals generated during CMP. Such signals have spectral and amplitude content which evolves during the course of the polish cycle. However, to date there has been no commercially available in-situ endpoint detection system using acoustic methods to determine end point.
Optical techniques are a common method of gathering data on the condition of a wafer""s surface during CMP. One such optical technique involves reflecting light off the surface of a wafer and capturing the reflected light by a properly positioned receptor. The receptor funnels the reflected light through a fiber optic cable to a desired metrology instrument that analyzes the data. The CMP tool is then able to act upon the analyzed data and may alter an operational parameter or terminate the planarization process.
Conventional optical techniques face many different technical challenges due to the hostile CMP environment. Wafers are typically planarized face-down on a polishing pad making it difficult to have direct optical communication with the wafer""s surface. In order to take measurements of the wafer""s surface, holes or transparent areas generally need to be manufactured into the polishing pad or the wafer may be allowed to travel over the edge of the polishing pad. However, allowing holes or transparent areas in the polishing pad, or polishing over the edge of the polishing pad, introduce variables into the CMP process that may not be desirable.
Another common problem for optical techniques is that slurry is generally used during the CMP process. The slurry may attenuate, or in some cases totally block, the optical signal. The optical signal must also typically travel through an air-slurry and slurry-wafer interface that distorts the measurements of the wafer""s surface.
Yet another problem for monitoring a wafer""s surface using optical techniques is that the wafer""s surface may have been built up by sequentially depositing multiple thin films on top of each other. Some of the deposited thin films may be made of oxides that are relatively transparent while other deposited thin films may be partially made of metals that are opaque to optical monitoring techniques. The problem arises that while desiring to monitor only the top thin film, optical techniques often receive so much noise produced by the lower thin film levels that it is difficult to make accurate measurements of the top thin film. In addition, the manufacturing process for semiconductors produces a nonuniform surface for the wafer whereby every point does not look like every other point on the wafer""s surface.
Yet another problem for using optical techniques to measure a wafer""s surface is that relative motion is required between the wafer and the polishing pad. Wafers are typically orbited, oscillated and/or moved linearly across a polishing pad while the polishing pad is typically simultaneously rotated, orbited or moved linearly. The constant relative motion between the wafer and the polishing pad makes it difficult to take repeatable measurements at the same point on a wafer""s surface.
Yet another problem with optical techniques is that the measurements of the wafer""s surface must be taken and analyzed quickly if the results are to be useful in the planarization process for the wafer. The optical measurements received from the wafer""s surface are often very complex and intermingled with noise from the slurry and/or lower thin film layers. Multiple algorithms generally need to be run on the optical measurements taken from the wafer""s surface before useful information is obtained. However, the planarization process for the wafer often requires the process to be stopped in real time with sub-second accuracy or the wafer may be under or over-polished.
Another approach for monitoring in-situ the change in thickness of a conductive thin film on a wafer""s surface is by indirectly inducing a current in the film and then detecting the current with a sensor comprising a capacitor and an inductor. As the thickness of the film changes (either increase or decrease), the changes in the current may be detected. Eddy currents are induced in the conductive film by generating an alternating electromagnetic field near the thin film. However, the signal strength of the endpoint signal may be affected in ways that are difficult to account for by wafers that have multiple metallized layers. In addition, the electromagnetic field may introduce harmful voltages and currents in the underlying active devices due to the penetrating nature of the magnetic field. While the non-contact nature of the measurement is desirable, the uncertainty in the location of the endpoint due to underlying metal traces and potential damage to the underlying active devices makes this approach problematic.
What is needed is a method and apparatus for detecting endpoint of a planarization process which is not significantly affected by a polishing pad blocking the view of a wafer""s surface, slurry covering the wafer""s surface, multiple opaque, transparent, or conductive layers on the wafer""s surface or movement between the wafer and the polishing pad. The monitoring system must also prevent damage to the wafer and perform quickly so that results produced can be used in real time.
Thus, it is an object of the present invention to provide a method and apparatus for determining the endpoint for a planarization process and/or to improve the planarization process.
One embodiment of the present invention has a working surface, for example a polishing pad, that is useful for planarizing a thin metal film that has been deposited on a wafer""s surface. A probe is positioned to take measurements of the wafer""s surface as the wafer""s surface is being planarized against the working surface. The probe may be fixed in position relative to the moving working surface, but is preferably embedded in the working surface so as to move in conjunction with the working surface. The probe may have two points for measuring resistance or impedance. However, it is preferable for the probe to have four points, two for inducing a current in the thin film and two for measuring a voltage between two points on the thin film within the current area. The resistance, impedance or voltage read may be used to determine certain characteristics regarding the wafer""s surface. The points on the probe, whether two or more are used, must be in electrical communication with the thin metal film on the wafer""s surface. The points may contact the thin metal film directly, use part of the working surface to complete the electrical connection and/or use a conductive solution introduced during the planarization process to complete the electrical connection. The conductive solution may take the form of conventional slurry, slurry that has been altered to have a desired resistance or a separately added fluid. The conductive solution may be introduced by pumping it through the working surface or by depositing it on the working surface.
The probe may be adapted to communicate with a metrology instrument that is able to analyze the signals captured by the probe. The probe may communicate with the metrology instrument using RF signals, electrical wires, optical paths (such as fiber optic cables) or other known communication techniques. The metrology instrument may be an ohmmeter or an impedance meter, but is preferably a combination current source and voltmeter. If the metrology instrument is a current source and voltmeter, the current may be DC, AC or a combination with the frequency for the AC in the range from a few Hertz to the megahertz range. However, the preferred current is AC in the kilohertz range.
The metrology instrument sends information regarding its measurements to a control system that is able to terminate or alter other parameters of the planarization process once the thin metal film on the wafer""s surface has reached a desired condition.
An example of the present invention in operation starts with a wafer being planarized against a working surface, such as a polishing pad. A probe may be embedded within the working surface so that as the working surface travels under the wafers surface, the probe also travels under the wafer""s surface bringing the probe into electrical communication with a thin film on the wafer""s surface. Two points on a four-point probe may be used to induce a current in the thin film and the other two points may be used to detect a voltage drop on the thin film within the area of induced current. The voltage drop may then be used to calculate the thickness or determine other parameters of the thin film on the wafer""s surface. Once the voltage or calculated thickness reaches a particular condition, the planarization process may be altered or terminated.
Another example of the present invention in operation is similar to the above described method. However, in this embodiment, the current induced in the wafer""s surface is used for more than creating a voltage drop between two points on the wafer""s surface. The current also affects the rate of plating and deplating of the thin metal film on the wafer""s surface. By altering the frequency and duration for inducing the current, the plating and deplating of particular areas of the thin film on the wafer""s surface may be controlled.